A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, for example, a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (for example, including part of, one, or several dies) on a substrate (for example, a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Generally, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The manufacture of ICs and other devices with a lithographic apparatus generally involves the replication of extremely fine sub-micron patterns that require an exceptionally high degree of positional accuracy. Thus, accurately positioning the movable components of the lithographic apparatus, for example, substrate handling components and patterning device handling components, is desirable. To accomplish such positioning, lithographic apparatuses typically use a multiple-degree-of-freedom positioning system that has one or more positioning modules to move the component and one or more position sensors to determine the position of the component. Typically, the sensors cannot discriminate between rigid body movement and non-rigid body movement of the component. Accordingly, the positioning system will detect motion of the component at the sensor location even though other portions of the component are not moving. For example, a sensor can detect motion of a patterning device support structure at the sensor locations even though the patterning device, which is coupled to the patterning device support structure, is not moving. In such cases, the lithographic apparatus will adjust the movement control of the patterning device support structure based on the detected motion, not the movement of the patterning device itself, creating real motion of the patterning device and positioning errors.
One source of non-rigid body movement is eigenmode vibrations. In a multiple-degrees-of-freedom positioning systems, an input along one axis (such as a force or position input) creates non-rigid-body eigenmode coupling motion along one or more other axes according to the eigenmode shape. This eigenmode coupling is not compensated for with existing compensation methods such as gain balancing and gain scheduling. Accordingly, there is a need for improved lithographic apparatuses and control methods that can compensate for such eigenmode coupling.